The Yellowstone and Grand Teton region is one of the most dynamic seismic areas in the world -- wracked by earthquakes, cracked by water boiling to the surface, and littered with the detritus of previous volcanic eruptions. Today, the bowels of the Yellowstone caldera are again filling with magma. Geologic studies show that, for the past 2 million years, the plateau has blown its top every 600,000 years or so -- and the last explosion was about 600,000 years ago. That means that a titanic blow -- bigger than anything seen in recorded history -- could happen, well, any century now, give or take thousands of years. The geological time frame is a long one, by human standards, but this didn't stop people from getting excited when an unprecedented "swarm" of minor earthquakes rattled the park in early 2009. The good news is that the big one is not imminent; geologists say things need to heat up considerably first.

As you'll learn when you visit the exhibits on the park's geology at Moose, Mammoth, and the various geothermal areas, what you see on the surface -- great layers of ash and the core of volcanic vents, such as Mount Washburn and Bunsen Peak -- is only a fraction of the story of Yellowstone and Grand Teton.

Situated on 2.2 million acres, Yellowstone is significantly larger than its sister to the south. Encompassing 3,472 square miles, Yellowstone boasts 310 miles of paved roads and 1,000 miles of backcountry trails, and it is home to more geysers and hot springs than the combined total in the rest of the world.

Although it can't match Yellowstone's size, Grand Teton National Park is nothing to sneeze at. It has towering mountain spires, which have been compared to cathedral towers, reaching almost 14,000 feet skyward; picturesque glacial lakes; and a great deal of interesting topography. The roughly 500 square miles of Grand Teton contain about 160 miles of paved roads and over 250 miles of hiking trails.

The Faces of Yellowstone National Park

By the end of the 1872 Hayden expedition, explorers had identified several distinct areas in the park, each with its own physical characteristics. Less spectacular than the craggy mountain scenery of Grand Teton, and less imposing than the vast expanses of the Grand Canyon in Arizona, Yellowstone's beauty is subtle, reflecting the changes it has undergone during its explosive past.

Although Yellowstone has its share of mountains, much of the park is a high mountain plateau. The environment changes dramatically as you ascend the mountain slopes from the foothill zones in the valleys -- the elevation at the entrance at West Yellowstone is 6,666 feet, for example, compared to 5,314 feet at the Gardiner entrance. Because the park lies about halfway between the equator and the North Pole, its summers consist of long, warm days that stimulate plant growth at the lower elevations.

As you walk the park trails, you'll find that plant distribution changes with the elevation. At the lowest elevations, down around 5,300 feet above sea level, you'll find grassy flats and sagebrush growing on dry, porous soils, with creeks and rivers cutting through to form wildlife-rich riparian zones. Next up: the foothills, sloping upward toward peaks, sometimes dotted by deposits of glacial moraine. Douglas fir, pine, and other conifers, as well as stands of aspen, clad these slopes, and there are marshes and ponds fed by the spring snowmelt. Shrubs and flowers, such as huckleberry and columbine, favor these wet, shady spots.

Then comes the mountain zone (6,000-7,600 ft.), thickening forests dominated by lodgepole pine, broken by meadows where deer, elk, and moose often graze. The transition area between the highest forest and the bare surface above timberline is known as the subalpine zone (7,600-11,300 ft.). Finally, we come to the bare rock at the very top of the continental shelf, where small, hardy plants, such as glacier lilies and sky pilot, bloom briefly after the annual thaw.

Although the park is most famous for its geysers, visitors can choose among very different environments, reflections of the long-term effects of geologic activity and weather.

The limestone terraces at Mammoth Hot Springs give testimony to the region's subsurface volcanic activity. The park sits atop a rare geologic hot spot where molten rock rises to within 2 miles of the Earth's surface, heating the water in a plumbing system that still mystifies scientists.

The northern section of the park, between Mammoth Hot Springs and the Tower-Roosevelt region, is a high-plains area that is primarily defined by mountains, forests, and broad expanses of river valleys that were created by ice movements.

The road between the Tower-Roosevelt junction and the northeast entrance winds through the Lamar Valley, an area that has been covered by glaciers three times, most recently during an ice age that began 25,000 years ago and continued for 10,000 years -- in geologic terms, just yesterday. Because this area was a favorite of Theodore Roosevelt, it is often referred to as "Roosevelt Country." The beautiful valley where elk, bison, and wolves interact is dotted with glacial ponds and strewn with boulders deposited by moving ice.

Farther south are Pelican and Hayden valleys, the two largest ancient lake beds in the park. They feature large, open meadows with abundant plant life that provides food for a population of bison and elk.

In the warm months, you'll enjoy the contrast between the lush green valleys and Canyon Country, in the center of the park. Canyon Country is defined by the Grand Canyon of Yellowstone, a colorful, 1,000-foot-deep, 24-mile-long gorge -- in many opinions, just as dramatic as its cousin in Arizona. The Yellowstone River cuts through the valley, in some places moving 64,000 cubic feet of water per second, and creating two waterfalls, one of which is more than twice the height of Niagara Falls.

When you arrive at the southern geyser basins, you might feel that you've been transported through a geologic time warp. Here you will find the largest collections of thermal areas in the world -- there are perhaps 600 geysers and 10,000 geothermal features in the park -- and the largest geysers in Yellowstone. The result: boiling water that is catapulted skyward and barren patches of sterile dirt; hot, bubbling pools that are unimaginably colorful; and, of course, the star of this show, the geyser Old Faithful. Plan on spending at least 80 minutes here, as that's the typical period between the eruptions that send thousands of gallons of boiling water through the sky at a speed exceeding 100 mph.

You'll see the park's volcanic activity on a 17-mile journey east to the lake area, the scene of three volcanic eruptions that took place more than 600,000 years ago. When the final eruption blasted more than 1,000 square miles of the Rocky Mountains into the stratosphere, it created the Yellowstone caldera, a massive depression measuring 28 by 47 miles, and Yellowstone Lake basin, some 20 miles long and 14 miles wide, reaching depths of 390 feet. The landscape here consists of flat plateaus of lava that are hundreds of feet thick.

Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.